This invention relates to a method of making microcapsules containing water-immiscible oil droplets and more particularly to an improved method of making multi-nucleus microcapsules formed by aggregation of such mono-nucleus microcapsules which are useful for the manufacture of pressure-sensitive copying papers.
Pressure-sensitive copying papers and heat-sensitive recording papers which utilize the colour developing reaction between electron donating organic chromogenic material (hereinafter referred to as "colour former") and electron accepting acidic reactant material (hereinafter referred to as "acceptor") are now widespread. In pressure-sensitive copying paper at least one of the colour former and the acceptor is contained in microcapsules so as to be isolated from the other and they become into contact with each other by rupturing such microcapsules to develop a colour. In a most typical type of pressure-sensitive copying paper minute oil droplets in which the colour former is dispersed or dissolved are encapsulated and coated onto papers.
Pressure-sensitive copying papers found their usefulness in a variety of commercial material applications. For example, they are very useful as computer output recording papers, business forms and copying slips. In these arts one of the requirements in the quality is that the copying can be done at one time and under usually applied pressure for as many superposed sheets of paper as possible. This requirement, however, involves a disadvantage that paper sheets are easily smudged by inadvertent pressure during storage, handling and shipping. There has been proposed pressure sensitive copying paper having a coating layer of multi-nucleus microcapsules, e.g., as disclosed in U.S. Pat. No. 3,041,289 and Japan Kokai (Laid Open Patent Publication) No. 118,509 of 1976. The conventional technique of making the multi-nucleus microcapsules has never been able to resolve the antinomic problem that the copying capacity of the multilayered copying paper sheets should be increased while preventing the copying paper sheets from being smudged by inadvertent pressure.
The most typical method for making oil-containing microcapsules is to utilize the coacervation technique. For example, according to the disclosure in U.S. Pat. No. 2,800,457 oil-containing microcapsules are made by the following steps:
(1) A mixture of two different hydrophilic colloid sols in which oil droplets are dispersed is prepared. The mixture may be made by forming an aqueous sol of one colloid material, emulsifying the selected oil therein, and mixing the emulsion with an aqueous sol of another colloid material, or the two sols may be made and mixed and the oil emulsified therein. The two colloid materials are gellable and have opposite electric charges. PA1 (2) Coacervation is caused by dilution and/or by adjusting the pH of the mixture to form and adhere to coacervate around each of the oil droplets. PA1 (3) The coacervate around each of the oil droplets is gelled by cooling; and PA1 (4) The coacervate is further hardened by addition of a hardening agent or by adjusting the pH to an alkaline zone.
It is considered that in order to resolve the before-mentioned antinomic problem it is imperative to obtain oil-containing microcapsules having selected and generally uniform particle sizes within a limited range. The particle size of the microcapsules obtained by the above mentioned coacervation technique depends on various factors such as the temperature and the pH of the system, the colloid concentration in the system, the kinds of colloid materials used and the composition ratio of the oil and the colloid materials. It is our conclusion after substantial investigations and experiments that it would be impossible or extremely difficult to obtain microcapsules having desired and generally uniform particle sizes with chemical adjustment of any of the above mentioned factors alone.
In the U.S. patent application filed by Makoto Miyake et al. on Jan. 27, 1978 with claim to the Convention Priority of Japanese patent application No. 9,022 of 1977 filed on Jan. 28, 1977, it has been proposed to control aggregation of particles of oil droplets each having a coacervate therearound by an agitation flow produced by extremely slowly rotating an agitator in a vessel containing the coacervate suspension whereby multi-nucleus capsules having desired diameters and desired particle size uniformity may be obtained owing to an appropriate balance between the adhesion force of coacervate and the separation force given by the control agitation flow. This method for controlling aggregation by agitation is advantageous because its operation is simple. However, we have found that formation of multi-nucleus microcapsules is highly affected by the viscosity of the solution for coacervation at a high temperature and the behaviour of coacervates toward gelation during the cooling step.
The primary object of the invention is to provide an improved method of making multi-nucleus oil-containing microcapsules having selected and generally uniform particle sizes as desired.
Another object of the invention is to provide an improved method of controlling the particle size and its uniformity of the multi-nucleus oil-containing microcapsules as desired through the utilization of a specified agitation flow during the steps of making oil-containing microcapsules according to the coacervation technique.
A further object of the invention is to provide an improved method of controlling the particle size and its uniformity of the multi-nucleus oil-containing microcapsules with use of specified colloid materials in addition to the utilization of the above mentioned agitation flow.
A still further object of the invention is to provide an improved method for the production of pressure-sensitive copying paper sheets which have an improved recordability for multi-layered or thick copying paper sheets while preventing those paper sheets to the utmost extent from being soiled or smudged by an inadvertent pressure applied.
Other objects and advantages of the invention will be apparent from the following detailed description.